1. Field of the Invention
The present invention relates to an oil pump system for a vehicle having a vane pump for supplying a hydraulic fluid to an external hydraulic actuator so as to serve as a hydraulic power source for the hydraulic actuator. More particularly, the present invention concerns an oil pump system for a vehicle having a vane pump for supplying a hydraulic fluid to a hydraulic actuator of an automatic transmission or the like provided in a vehicle such as an automobile, so as to serve as a generating source for the hydraulic actuator.
2. Related Art
A vehicle such as an automobile is provided with an oil pump system for a vehicle which includes a hydraulic pump for supplying a hydraulic fluid to a hydraulic actuator of an automatic transmission or the like provided in a vehicle such as an automobile so as to serve as a generating source of hydraulic pressure required for the hydraulic actuator, as well as a controller for controlling the operation of the hydraulic pump.
To described an example of a system which is provided in an automobile having an automatic transmission among the oil pump systems for a vehicle, referring to FIG. 6, the oil pump system for a vehicle is comprised of a hydraulic pump 5 which is driven by the torque of a rotating shaft (crankshaft) of an engine 50 and is adapted to supply to an automatic transmission 51 a hydraulic fluid for its speed ratio control through a piping with a check valve 7b provided therein. This system is further comprised of an electric power-driven oil pump (electric power-driven assist pump) 10 which is made up of a pump 12 and an electric motor 13 for driving the pump 12, so as to assist the hydraulic pressure of the hydraulic fluid during an idling stop of the automobile through a piping with a check valve 7b provided therein on the basis of a command from an engine control unit (ECU) 4 serving as the controller. This oil pump system for a vehicle is further comprised of a relief valve 7r disposed between a discharge portion of the pump 12 and the check valve 7b and adapted to relieve the hydraulic fluid into a drain when the discharge pressure of the electric power-driven oil pump 10 has assumed an overpressure state. Each of the hydraulic pump 5 and the electric power-driven oil pump 10 is adapted to suck the hydraulic fluid stored in an oil pan 52 provided below the engine 50 and supply (force feed) it to the automatic transmission 51.
As the hydraulic pump 5 of this type, a so-called vane pump, which is a kind of rotary volumetric pump, is frequently used since although it has a compact and simple structure, it has excellent characteristics of large capacity, low noise, and low pulsation (refer to JP-A-2000-18377).
The aforementioned vane pump, although not shown, includes a rotor, a plurality of vanes projecting retractably from a plurality of vane accommodating grooves provided in the rotor, and an elliptical cam ring surrounding the rotor. Further, as the rotor is rotated inside the cam ring, the vanes are adapted to project by the action of the centrifugal force due to the rotation of the rotor and by the hydraulic pressure on the discharge side of the vane pump so as to slide on a cam surface at the inner surface of the cam ring. At this time, a plurality of pump chambers, which are partitioned by the vanes and accommodate the hydraulic fluid, are formed between an outer periphery of the rotor and the cam surface. Among the pump chambers, those whose volume is large become high-pressure portions where the hydraulic fluid is set under high pressure, while those whose volume is small become low-pressure portions where the hydraulic fluid is set under low pressure.
Then, the hydraulic fluid is sucked from the oil pan 52 by the low-pressure portions, and the sucked hydraulic fluid is fed with pressure to the automatic transmission 51 by the high-pressure portions.
In addition, in the electric power-driven oil pump 10, in a rotation range of the engine 50 at an idling speed Na or below, the number of revolution of the electric motor 13 is controlled by the ECU 4, such that a total discharge rate, which is a total of a discharge rate of the vane pump and a discharge rate (auxiliary flow rate) of the electric power-driven oil pump 10, becomes a fixed rate irrespective of an engine speed (rotational speed of a rotating shaft of the engine 50).
However, in such an automobile, at the point of time when the automobile is started by starting again the rotation of the engine 50 from a standstill state, i.e., from the idling stop state with the rotation of the engine once stopped, the rotor does not rotate in the vane pump, and the vanes are in a state of being not in contact with the cam surface of the cam ring. For this reason, the hydraulic fluid is not sealed in the pump chambers formed by the plurality of vanes between the outer periphery of the rotor and the cam surface, and the low-pressure portions and the high-pressure portions are not formed, so that the hydraulic fluid is not in a state of being fed with pressure to the automatic transmission 51 of the automobile.
For this reason, conventionally, referring to FIG. 5, when the engine 50 is started from the idling stop state, there occurred an engine rotation range (the range of an engine speed N=0 to No (min−1) in FIG. 5) where, despite the above-described number-of-revolution control of the electric motor 13 by the ECU 4, a total discharge rate QT′ (=Qa+Qb′) (m3/min) does not become a fixed rate, and a flow rate Q of the hydraulic fluid supplied from the vane pump to the automatic transmission 51 is lower than a required flow rate Qn. Namely, the discharge rate Qb′ of the conventional vane pump exhibits a downwardly convex curve in the engine rotation range, and a power loss (corresponding to the area of S0 in FIG. 5) conventionally occurred. There have been cases where this power loss constitutes a hindrance to the smooth starting of the automobile. This also leads to a decline in the reliability of the vane pump as the hydraulic pump 5 for an automobile.
It should be noted that it is not preferable to make a large-scale structural change in the oil pump system for a vehicle to overcome the above-described problem of a hydraulic pressure shortage of the hydraulic fluid which is fed from the vane pump to the automatic transmission 51 during the starting of the engine 50, since this correspondingly requires a change in the manufacturing process of the automobile and results in an increase in the number of parts used.